1. Field of the Invention
The present invention relates to an ultrasonic flaw detection method for detecting a defect present in a test object by using an ultrasonic probe, and an ultrasonic flaw detection apparatus employing this method and, more particularly, to a method and apparatus for performing ultrasonic flaw detection, in which a peak frequency and a frequency bandwidth of an echo signal obtained from the ultrasonic probe attached to the object are independently controlled.
2. Description of the Related Art
In an ultrasonic flaw detection apparatus for detecting defects or flaws present on the surface of or inside a steel plate by using an ultrasonic wave, an impulse signal obtained by utilizing discharge characteristics or the like of, e.g., a charge circuit is applied to an ultrasonic probe attached to the surface of the object. An ultrasonic wave is transmitted from the ultrasonic probe to the inside of the object. When a defect is present, upon propagation of the ultrasonic wave inside the object, a reflected wave is generated. The ultrasonic probe detects this reflected wave and outputs it as an echo signal. The level of the echo signal output from the ultrasonic probe corresponds to the size and shape of the defect. Therefore, the presence/absence of the defect and its size and shape are detected in accordance with this signal level.
In this conventional apparatus, eve if flaw detectors having the same technical specifications and ultrasonic probes having the same technical specifications are used, and even if the same defect is detected by these components, if a plurality of channels are present, the same results cannot be expected between the channels due to the following reason. The properties of an object are not always uniform, and ultrasonic probes do not necessarily have identical characteristics. For these reasons, differences occur between the characteristics of the channels. As a result, frequency characteristics such as operating frequencies and frequency bands of the respective channels are fixed to different values.
A wide-band probe has advantages in that the echo width of a received echo signal is sharp, and that an S/N ratio obtained with an attenuating material is higher than that of a narrow-band probe. Since the electrical impedance of this wide-band probe, however, is low, the differences in characteristics have large influences on output waveforms. Variations in frequency characteristics such as operating frequency characteristics and frequency bandwidths therefore occur between the respective probes. In a multichannel ultrasonic flaw detection apparatus having a probe array consisting of a plurality of ultrasonic probes, it is impossible to obtain a uniform flaw detection sensitivity of the object as a whole.
In addition, when an ultrasonic probe is replaced with a new one, differences in characteristics between the ultrasonic probes are present. For this reason, adjustment must be performed in accordance with these differences in characteristics and deterioration over time in each ultrasonic probe. The adjustment operations are time-consuming and cumbersome and require much labor.
An ultrasonic flaw detection method called a CS method (Controlled Signal Technique) is proposed in which an impulse signal is not applied to an ultrasonic probe. According to this CS method, as shown in FIG. 11A, a tone-burst pulse signal obtained by extracting a carrier wave having a predetermined carrier frequency f.sub.c every predetermined time interval is applied to the ultrasonic probe. Some techniques of the CS method are introduced in Published Unexamined Japanese patent Application Nos. 62-180267 and 62-54160 and Published Examined Japanese Patent Application No. 59-10501.
The tone-burst pulse signal has frequency characteristics as shown in FIG. 11B. Frequency components in a narrow band centered on a peak frequency f.sub.P are present, as shown in FIG. 12. For the sake of comparison, a characteristic curve indicated by a dotted line is a frequency characteristic curve obtained when an impulse signal is applied. When the carrier frequency f.sub.c of the impulse signal to be applied to the ultrasonic probe is changed, the peak frequency f.sub.P of the echo signal changes accordingly. For example, when the carrier frequency f.sub.c of the pulse signal is changed in an order of 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, and 7 MHz, the peak frequency f.sub.P of the echo signal changes in an order of 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, and 7 MHz accordingly, as shown in FIG. 13.
As the cycle count N representing the number of waves included in the pulse signal is increased, the ratio of the components of the carrier frequency f.sub.c to other frequency components of the pulse signal is increased. As a result, the bandwidth of the pulse signal is narrowed, and a steep frequency characteristic curve is obtained. FIG. 14A shows a pulse signal waveform for N=1. FIG. 14B shows a frequency characteristic curve of the pulse signal shown in FIG. 14A. FIGS. 15A and 15B show a signal waveform and frequency characteristics for N=5. FIGS. 16A and 16B show a signal waveform and frequency characteristics for N=10.
It is thus understood that the bandwidth of the pulse signal is decreased with an increase in cycle count N of the pulse signal. When the cycle count N is changed, the frequency bandwidth W of the ultrasonic waves applied to the object can be changed to an arbitrary value.
In practice, a uniform flaw detection sensitivity in the entire detection range of an object in a multichannel ultrasonic flaw detection apparatus having an array of a plurality of ultrasonic probes must be realized. At the same time, adjustment operations based on the differences in characteristics upon replacement of a probe and the deterioration over time in each probe must be performed. It is very difficult to control that the frequency characteristics such as the peak frequency f.sub.P and the frequency bandwidth W of the echo signal output from the ultrasonic probe satisfy the above conditions.